1. Field of the Invention
The present invention relates to a negative electrode for a power storage device and a method of manufacturing a power storage device using the negative electrode.
2. Description of the Related Art
In recent years, power storage devices such as lithium secondary batteries, lithium-ion capacitors, and air cells have been developed.
A power storage device has two electrodes: a positive electrode and a negative electrode. To provide a negative electrode for a power storage device, an active material is formed over a surface of a current collector. As a negative electrode active material, a material that can occlude and release ions functioning as carriers (hereinafter referred to as carrier ions), such as carbon or silicon, is used. Silicon or phosphorus-doped silicon, for example, is beneficial to the higher capacity of a power storage device because they can occlude about four times as many carrier ions as carbon and accordingly have higher theoretical capacity than carbon.
However, as the number of carrier ions to be occluded increases, occlusion and release of carrier ions in charge/discharge cycles changes the volume of an active material significantly to impair adhesion between a current collector and silicon; thus, charge/discharge degrades battery characteristics. In view of this, the following way has been proposed: a layer containing silicon is formed over a current collector and a layer containing a graphite is formed over the layer containing silicon so that expansion and contraction of the layer containing silicon influences less on degradation of battery characteristics (see Patent Document 1).
Further, since silicon has lower electric conductivity than carbon, surfaces of silicon particles are covered with a graphite and the active material layer including the silicon particles is formed over a current collector, whereby a negative electrode having an active material layer the resistivity of which is reduced is manufactured.
In recent years, the use of a graphene as an electronic material having electric conductivity in semiconductor devices has been considered. The term graphene refers to a sheet of carbon molecules with a thickness of one atomic layer having sp2 bonds.
Since a graphene is chemically stable and has favorable electric characteristics, it has been expected to be applied to semiconductor devices, for example, channel regions of transistors, vias, and wirings. In addition, the following way has been proposed: active material particles are covered with a graphite or a graphene in order to increase the electric conductivity of an electrode material for a lithium-ion battery (see Patent Document 2).
Further, the following way has been proposed: in a power storage device including a positive electrode and a negative electrode which are each provided with a plurality of protrusions for higher capacity, an insulator is formed at a top portion of each of the protrusions of the positive and negative electrodes in order to reduce a pressure which is applied to a separator between the electrodes when the electrodes increase in volume due to charge/discharge (see Patent Documents 3 to 5).